An updated regional three-dimensional (3-D) lithostratigraphic model of the Paleozoic bedrock of southern Ontario has been produced using Leapfrog© Works software, improving a model completed in 2019. The model encompasses the entire Phanerozoic succession of southcentral and southwestern Ontario consisting of approximately 1500 metres of Paleozoic bedrock and an area of 110,000 km2. Fifty-three Paleozoic bedrock layers representing 70 formations, as well as the Precambrian basement and overlying unconsolidated sediment, were modelled at a spatial resolution of 400 m. Petroleum well records in the Ontario Petroleum Data System (OPDS) were the principal data source, supplemented by Ontario Geological Survey (OGS) deep boreholes, measured sections, control points, Michigan boreholes, and select bedrock provincial water well records. The model format can readily support numeric groundwater-flow modelling. From 2019 to 2020, project geologists and data support staff of the Oil, Gas and Salt Resources Library (OGSRL) completed edits to 17,595 formation tops in a total of 3,419 wells, resulting in a revised data set and permanent improvements to the petroleum well database. Formation top data from a total of 20,836 Ontario petroleum wells, 199 OGS stratigraphic tests, 15 measured sections, 3 Michigan petroleum wells, and 30 control points were utilized, including seven new control points added to improve layer extrapolation beneath Lake Huron. The new model improves the resolution of the subcrop surface and there is a more accurate and realistic rendering and correlation of the topography and bedrock geology of the Niagara Escarpment. Many anomalous outliers and structural and thickness anomalies in model layers have been identified and removed and gaps in the model are reduced. A model layer of the Salina D Salt has been added. There was a focus on improving the data quality and quantity for the formations of the Lockport Group in support of improved bedrock model layers and future hydrostratigraphic modelling. New features added to the model include: 3-D volumes of salt beds leased for underground mining at Ontario's 2 salt mines, solution-mined caverns in salt beds including those constructed for storage of liquefied hydrocarbons and petrochemicals, two-dimensional representations of oil and natural gas reservoirs, regional faults, and lithotectonic boundaries in the Precambrian metamorphic basement.